Reuse same browser for all pages in rasterize_and_record_micro

Don't restart the browser between each tested page in
rasterize_and_record_micro. This improves stability and -- somewhat
surprisingly -- reduces noise in the results. As an added bonus the
rasterize_and_record_micro.key_silk_cases test completes in about 9 min
compared to an earlier 16 min.

To measure the impact of the change I ran 18 rasterize_and_record_micro
iterations for 19 pages in key_silk_cases on a Nexus 5 (strapped to a
water bottle for cooling) and logged the per-page record_time results
for each run. The results were:

With browser restarts (baseline):
- Average record time: 1.78 ms
- Average record time standard deviation: 0.0978

Without browser restarts (patch):
- Average record time: 1.63 ms
- Average record time standard deviation: 0.0379

BUG=355952

Review URL: https://codereview.chromium.org/306923007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@273984 0039d316-1c4b-4281-b951-d872f2087c98

tools/perf/measurements/rasterize_and_record_micro.py

@@ -11,7 +11,7 @@ from telemetry.page import page_test
 
 class RasterizeAndRecordMicro(page_measurement.PageMeasurement):
   def __init__(self):
-    super(RasterizeAndRecordMicro, self).__init__('', True)
+    super(RasterizeAndRecordMicro, self).__init__('')
     self._chrome_branch_number = None
 
   @classmethod